PE and PP are immiscible polymers. Without proper compatibilizers or tie resins, coextrusion of these materials can lead to separation of the two base polymers. Heretofore, measures to improve the bonding between the polymers include: using PP and PE blends in place of pure PP or PE; using PE and ethylene acrylate copolymer blends in place of pure PE; or using various tie resins which are compatible with both PE and PP during coextrusion.
Using a blend of PE and PP can improve their compatibility to some degree. However, it is difficult to achieve a good balance between improving bonding and retaining desired properties. Blending PE with PP affects the end blended product's optical, mechanical and barrier properties.
A blend of PE with ethylene methacrylate (EMA) can improve its adhesion to PP. Nonetheless, a large percentage of EMA is required to achieve a good adhesion. It inevitably increases material cost and affects the properties of PE. It has also been observed that at a high percentage of ethylene methacrylate copolymer, the blend adheres not only to PP, but also to the equipment. This can lead to frequent disruptions to the production and an increase in downtime. The same problem exists when using ethylene methacrylate copolymer as an individual tie layer between PP and PE.
Polypropylene based POP (Polyolefin Plastomer) and POE (Polyolefin Elastomer) using metallocene techniques can produce copolymers with excellent compatibilities to both PE and PP, and thus are effective tie resins for the two. Nonetheless, POP and POE typically have lower crystallization temperatures than PP and PE. It has been observed that this difference in crystallization temperature can cause wrinkles in multilayer films during sealing.
Kiang (U.S. Pat. No. 5,336,721) discloses an adhesive consisting of an ethylene alkyl acrylate copolymer and a polypropylene and ethylene copolymer grafted with carboxylic acid or anhydride. The percentage of the latter is between 3 to 10 percent. The main purpose of this adhesive blend is for the adhesion between PMP and EVOH.
Adur (U.S. Pat. No. 4,957,968) discloses an adhesive consisting of a polyolefin modified with functional groups, including anhydride and capable of binding metals, an unmodified polyolefin and a partially cured elastomer. The percentage of the modified polyolefin is in the range between 15 to 40 percent. The main purpose of this adhesive is to promote adhesion between polymer and metals.
Lee (U.S. Pat. No. 8,236,886) discloses an adhesive composition comprising a polymer grafted with carboxylic acids or anhydrides, a paraffinic liquid hydrocarbon and a polyolefin base resin of homopolymer, copolymer or mixtures thereof. However, there remains room for improvement and variation within the art.